1. Field of the Invention
The present invention is directed to an LED lamp and a driving circuit to drive an LED array. More particularly, the present invention is directed to an LED lamp and a driving circuit which can drive an LED array with a compensation for conditions which change luminous output of the LED array. This invention can find particular application where the LED array is utilized in a device such as a traffic signal or another indicating signal.
2. Discussion of the Background
The use of LED arrays in indicating devices, such as traffic signals, is known. One drawback with using LEDs in an indicator such as a traffic signal is that luminous output of an LED degrades with both time and increasing temperature. For red LEDs degradation with respect to temperature will typically result in a loss of approximately one percent of intensity of the LED with every one degree centigrade increase in temperature. Conversely, as temperature decreases, intensity of light output by an LED increases. Moreover, LEDs gradually degrade over time, and thus become dimmer as they get older.
One known system senses a temperature at the LED or senses a light output at the LED, and utilizes the sensed temperature or sensed light output as a feedback to a power supply. Such a system is disclosed in U.S. Pat. No. 5,783,909 to Hochstein. This patent discloses (1) sensing either temperature at an LED or intensity output of an LED, (2) feeding back the sensed temperature or intensity to a power supply, and (3) then increasing or decreasing an average current output by the power supply based on any increase or decrease in temperature at the LED or any increase or decrease in the light output of the LED.
One drawback with such a system as disclosed in Hochstein is that such a system may not operate properly at low temperatures. As a specific example, a traffic signal is normally switched on and off by solid state relays. These relays may have a minimum current below which the relays cannot operate reliably. Utilizing a feedback operation such as in the device of Hochstein results in the following problems during low temperature operation of the LED array.
Because of the feedback operation in the device of Hochstein, at a low temperature a small total current is supplied to drive an LED array since the LED array is very bright at the low temperature. The total current supplied to the LED array may as a result cause the current through the load switch to fall below the minimum current required for the solid state relays to properly operate. In traffic signals it is also desirable to reduce lamp intensities at low temperatures while maintaining an input current to be compatible with a lamp controller. The device of Hochstein does not address problems of controller compatability.